Trifid was developed by Félix Delastelle in 1902 as an improvement to his bifid cipher.
Trifid also combines substitution with fractionation and transposition. The main difference is the substitution of letters by trigrams, i.e. the use of three 3×3 squares with an alphabet of 26 letters and one character.
Encrypting with Trifid
First, the alphabet is used to create the three 3×3 squares. GC Wizard offers the two standard alphabets A-Z and Z-A as well as a user-defined alphabet.
The plain text is translated character by character into trigrams, which are written in three lines one below the other.
Now the text is divided evenly into blocks. Delastelle suggests 5 or 7 characters for this. The last block can be shorter.
In the third and last step, the digits of the blocks are combined into trigrams from top to bottom/left to right and encoded into letters.
Decrypting with Trifid
After creating the three 3×3 squares, the ciphertext is translated character by character into trigrams.
These are divided according to the block size and each block is written down in three lines.
In the third and last step, trigrams are formed from each of the three lines and ciphered into letters.